1. Field of the Invention
The invention concerns an apparatus having an automatic firing arrangement.
Such apparatuses are suitable for example for controlling combustion in high-output heat generating installations which are operated with liquid or gaseous fuels.
2. Description of the Prior Art
Automatic firing arrangements which are used in such apparatuses are known for example from the Landis & Gyr company publication "Feuerungsautomaten fur Ol und Gasbrenner", L..,. An air fan, a fuel pump (for example an oil pump), a fuel valve and an ignition device are controlled by means of an automatic firing arrangement of that kind. That assembly can control and monitor both the procedure for starting up a burner and also operation thereof subsequently to such a start-up procedure.
In the known apparatuses an approximately constant flow of air is produced by means of an air fan, and the flow of air is influenced by means of an air flap disposed at an upstream or downstream position, in such a way that the desired flow of air to the burner is produced. For safety reasons, the apparatus uses air pressure monitors which, on the basis of the increased pressure which is linked to the flow of air, establish whether a given minimum amount of air is flowing therepast. Regulation of the amount of air which is fed to the burner, by means of the air flap, is difficult because its characteristic is severely non-linear precisely in the low-load range. In a modern oil or gas burner therefore the speed of rotation of the fan motor must be controllable over a wide range.
An apparatus having an automatic firing arrangement as is described in the European patent application EP 614048 is suitable for use in relation to burners of low to medium output. In such burners, the fan drive employed is a controllable-speed DC-motor with integrated electronic control system and integrated sensor for detecting the speed of rotation, wherein provided on the motor beside the voltage supply connection are a control and a feedback signal connection for the reference value and the actual value respectively of the speed of rotation.
The drive for the air fan in high-output burners is generally an asynchronous motor which for example is operated over three phases in an electrical star circuit directly from the mains. As only slight air pressure differences are formed in the low-load mode of operation at a low speed of rotation of the fan drive, because of the low air through-put, it is scarcely possible to adjust air pressure monitors in such a way that they provide a reliable and fail-safe response at a low-load condition. In addition the statically operating air pressure monitors cannot be tested for correct operation when the burner is operating in the continuous mode.
The speed of rotation of the fan drive can be detected for example by means of Hall sensors. Such a design suffers from the disadvantage that magnetic markings have to be disposed on the motor shaft or on the fan impeller. The use of external sensors and transmission of the sensor signals to the automatic firing arrangement also requires a separate feedback signal line.
The speed of rotation of a motor can also be detected by means of tachogenerators, in which case the drive shaft must also be extended out of the motor on the side which is opposite to the fan impeller. Such motors are generally only obtainable as a special design and are correspondingly expensive. In addition the output signal of a tachogenerator contains no information about the direction of rotation.